In typical biofabrication and 3D bioprinting methods, layers of cell-laden hydrogel structures called bioinks are extruded to build functioning biological tissues. But using bioresins laden with cells, instead of bioinks, for lithography bioprinting methods, like digital light processing (DLP) and stereolithography (SLA), can be used to create patterns that are even more intricate than those possible with extrusion-based 3D printing; these patterns also do a better job of mimicking the complex architecture of tissue.

The team’s DLP technique projects a patterned mask of UV or visible wavelength light, with a digital micro-mirror device, onto the bottom surface in a polymer resin bath. Once they’re exposed to this light, certain regions of the resin are polymerized, and the platform moves up to create a new layer.

Two different types of hydrogel (PVA-MA and Gel-MA) make up the bioresin, along with a photoreactive compound which, when illuminated with visible light, initiates a chemical reaction. Over 90% of the bioprinted cells created with this technique survive up to three weeks once the resin is biofunctionalized with 1 wt% Gel-MA; additionally, the seeded cells successfully attach to the gel and spread across it.

The abstract reads, “While significant progress has been dedicated to the development of cell-laden bioinks for extrusion-based bioprinting, less attention has been directed towards the development of cyto-compatible bio-resins and their application in lithography-based biofabrication, limiting the advancement of this promising technology. In this study, we developed a new bio-resin based on methacrylated poly(vinyl alcohol) (PVA-MA), gelatin-methacryloyl (Gel-MA) and a transition metal-based visible light photoinitiator. The utilization of a visible light photo-initiating system displaying high molar absorptivity allowed the bioprinting of constructs with high resolution features, in the range of 25–50 μm. Biofunctionalization of the resin with 1 wt% Gel-MA allowed long term survival (>90%) of encapsulated cells up to 21 d, and enabled attachment and spreading of endothelial cells seeded on the printed hydrogels. Cell-laden hydrogel constructs of high resolution with complex and ordered architecture were successfully bioprinted, where the encapsulated cells remained viable, homogenously distributed and functional. Bone and cartilage tissue synthesis was confirmed by encapsulated stem cells, underlining the potential of these DLP-bioprinted hydrogels for tissue engineering and biofabrication.”

According to biomedical engineer Dr. Khoon Lim, a research fellow at the University of Otago Christchurch and the lead author of the study, the bioresin’s macro-component is a mix of both biological and synthetic polymers.

“Lithography-based fabrication technologies have long been used in jewellery making and in the automotive industry, for example, using a range of commercially-available resins. These resins often contain organic solvents or toxic chemicals and require photo-initiators that are only soluble in toxic organic solvents. To make our resins ‘bio’, we employed a combination of macromers (photo-responsive PVA-MA and Gel-MA) and the photo-initiator ruthenium. All these components are water soluble and not cytotoxic to cells,” explained Dr. Lim.

“The PVA-MA has versatile physical and mechanical properties that we can tailor with no batch-to-batch variation. And we know that PVA hydrogels are good candidates here because they have previously been used for multiple tissue engineering applications, including neuronal, cartilage and bone.”

The photo-initiator used in the bioresin is very efficient at the custom 400-450 nm wavelength of a commercial DLP 3D printer. This lets the researchers produce accurate hydrogel structures that feature excellent spatial resolution.

Woodfield explained, “It is the combination of all these components that allows us to fabricate biofunctional hydrogels with physico-mechanical properties that can be tuned to different tissue engineering applications.”

DLP printing allowed biocompatible hydrogels to be fabricated into high-resolution 3D patterns and architectures that are not achievable with extrusion-based printing systems. These include woven mats (top), and chain-mail designs.

Woodfield also said that topological features like pillars and gratings can be easily created on the surface of DLP 3D printed constructs, since the team’s method offers such high resolution. The cells can also be successfully embedded within the bioresin constructs without settling, and with high cell viability. So far, the team has used their new bioresin to synthesize both bone and cartilage tissue.

“These features provide the physical as well as spatial cues needed to control cellular behaviour – something, again, that we cannot easily achieve with extrusion bioprinting. We can also fabricate convoluted structures such as intrinsic vascular networks or cell-laden macro- and microfluidic devices,” said Woodfield.

“We now plan to try it out in different applications, such as making liver models and cancer models for high-throughput drug testing and vascular engineering.”